Electrical discharge device



June 21, 1949. c. G. SMITH ELECTRICAL DISCHARGE DEVICE Filed Nov. 50, 1945 IN VEN TO]? CHARLES 6. SMITH A TV Patented June 21, 1949 ELECTRICAL DISCHARGE DEVICE Charles G. Smith, Medford, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application November 30, 1945, Serial No. 632,072

12 Claims.

This invention relates to an electrical discharge device and associated system of the type intended to convert direct current into alternating current of a desired frequency.

In order to avoid heavy and expensive apparatus, such as rotary converters usually required to convert direct current into alternating current, it has heretofore been proposed to utilize gaseous discharge devices having a plurality of main anodes, and to commutate the discharge from anode to anode in order to alter the direction of flow of the current through external circuits. Such constructions have not heretofore been commercially successful in the conversion of currents of high amperage and voltage due to the fact that the commutation has heretofore been secured by the action of an electromagnetic field upon the current stream in the discharge space of the tube. Such action of the magnetic field upon the discharge stream or current through the evacuated space of the tube is likely to diiiuse the discharge so that it assumes the form of a distributed discharge. Under these conditions commutation of the discharge from one anode to an adjacent anode is not sharp and definite and may, therefore, result in unsatisfactory operation or in some cases in cessation of operation.

It is among the objects of th present invention to provide a construction of the type described in which the commutation of the discharge from one anode to another is eifected by operating magnetically upon the are spot of a mercury pool type cathode rather than upon the discharge stream between the cathode and anode.

A further difllculty, arising in connection with prior art devices of th type to which the invention relates, resides in the fact that at high voltages, due to the difference in potential between adjacent anodes, discharges might occur therebetween due to the high ionization existing in the discharge space at the instant of commutation of the discharge from one anode to another.

In the present invention such difliculty is obviated by the provision of a construction in which the distance between the successive anodes is much greater than the distance between the mercury pool cathode and any one of said anodes.

The foregoing and other objects and features of the invention will be made fully apparent to those skilled in the art from a consideration of the following detailed description taken in conjunction with the accompanying drawing in which:

Fig. 1 represents a top plan view of a tube con ports 8 of the anodes 9 and 9. A body of mercury l0 provides a mercury pool type cathode coacting with all of the anodes 9 and 9'. An auxiliary anode ll above the mercury pool cathode lll extends through the annular discharge space providedby the tubular envelope 6 and is supported by rods I! in a position above the mercury pool cathode 10. The auxiliary anode II also extends between the surface of this pool and each of the anodes 9 and 9'.

A magnet iii of U-shaped cross-section and having an annular energizing coil I4 is provided with a pair of annular pole pieces 15 and it, one of which lies within the annulus defined by the envelope 6 and the other l6 of which is positioned adjacent the outer circumference of the annular envelope 6.

The surface of the mercury pool in lies at or only slightly below the upper surface of the annular pole pieces 15 and It so that the magnetic field between the pole pieces l5 and I6 is somewhat arched in the region of the surface of the mercury pool cathode l0.

Alternate anodes 9 are connected to one minal ll of the primary winding l8 of a transformer l9 having a secondary winding 20 and the intervening anodes which are designated 9' are connected to the other terminal 2| of said primary winding l8. A condenser 22 is connected across a the end terminals of the primary winding I 8, and

a center tap 23 on said primary winding is connected to the positive terminal 24 of a source of direct current which it is desired to convert into alternating current of a definite frequency. The negative terminal 25 of said source of direct current is connected to the cathode pool III. The auxiliary anode l I is connected at one of the support rods l2 to a source of positive potential which may be derived from the source of direct current across the main cathode-anode circuit at the terminals 24 and 25 through a current limiting resistor 26.

The operation of the construction described in the foregoing is as follows:

ter-' auaaao Upon the application of a suitable potential.

picked up by the auxiliary anode H to form a continuous are between the auxiliary anode and the mercury pool cathode. In the instance shown a resistance-immersion ignitor 21 is provided to initiate the are spot. It will be understood that various ignition means for initiating the are spot upon the surface of the mercury pool cathode III are known in the art and that any such ignition device suitable for conventional mercury pool type tubes will be suitable for use in conjunction with the tube of the present invention. The discharge between the auxiliary anode II and cathode l having been initiated, the discharge between the auxiliary anode II and the cathode pool will be confined to a limited region of the tube extending in a more or less direct path between a relatively small cathode spot on the surface of the pool and some point on the anode ii adjacent such spot. If there were no magnetic field extending across the surface of the pool the are spot would travel about the surface of the pool in a haphazard manner and might move to the interior wall of the envelope 8 on either side of the mercury pool l0. Also, the are spot would tend to move over the circular path provided by the annular form of the mercury pool within the tube, but such movement would be non-uniform since the are spot might move irregularly in either direction. With a uniform magnetic field extending across the discharge space in a direction transverse to the lines of current flow between the auxiliary anode II and the cathode ii), the arc spot moves at a uniform speed in a direction normal to the lines of force of the magnetic field. The are spot thus rotates about the generating axis of the annulus 8 and the slight arching of the magnetic field due to the position of the surface of the mercury pool adjacent the upper edges of the pole pieces l and [6 causes the are spot to stay in or near the center of the pool. Also, when the field is acting in the region of the surface of the pool, rather than in the region between the anodes and the cathode pool, the field acts with reference to the are spot on the surface of the pool and not with reference to the lines of current flow between the are spot and the anodes. The effect of the field upon the are spot is to drive the spot in a direction counter to the direction of ordinary motor action. In other words, considering the are spot as the movable element, it is driven counter to the direction of rotation of a homopolar or unipolar motor. This motion of the are spot in a direction counter to motor action will be called retrograde motion in contradistinction to the motion which would occur if the anodes were spaced at some distance from the cathode and the magnetic field applied across this space. Thus, with the field extending in the direction shown, the direction of movement of the are spot would appear clockwise to the observer of Fig. 1.

The velocity of the are spot in a tube where other conditions are constant will depend upon the strength of the magnetic field provided that the strength of the field is not above a critical value which will occur in the region of 5,000 gauss. Below this value the speed. of the are spot will vary substantially linearly attaining values of about 100 meters per second in the region of the upper critical value.

.4 When the cathode-anode spacing is of the order of a few millimeters or greater, the pressure within the tube should not be greater than of the order of a centimeter of mercury, since above this upper limit of pressure the motion of the arc no longer is retrograde. If, however it is desired to utilize pressures above the order of a centimeter of mercury, then the electrode spacing must be decreased below the order of a few millimeters. I have found that with decreasing spacing the upper limit of pressure at which retrograde motion ceases increases.

As the are spot moves into the region adjacent one of the main. anodes 9 or 8', the discharge is transferred from one main anode 9 or 8 to another main anode. As the are spot moves away from one of the main anodes the discharge between the main anode and the cathode I0 is discontinued. However, the are spot continues to be maintained by a discharge between the auxiliary anode Ii and the cathode l0. When the are spot moves past one of the main anodes 9, current from the positive terminal of the direct current source flows from the center tap 23 on the primary winding I! to the end terminal I"! of said primary winding and thence to that one of the anodes 9 which the are spot is approaching. Accordingly, an alternation will be induced in the secondary winding 20 of the transformer I8. After the arc spot has passed the anode 9 and moved to a position adjacent the next succeeding anode 9', a current will now flow from the center tap 23 to the other end terminal 2| of the primary winding l8. Thus, the current through the primary winding 18 is, on this occasion, in the opposite direction to the flow of current through the winding on the immediately preceding occasion, and an alternation will now be induced in the secondary winding 20 in the opposite direction to the current previously induced therein. Accordingly, alternating current may be taken from the end terminals of the secondary winding 20. Since the anodes 9 and 9 v are symmetrically spaced around the circle defined by the envelope 6, and since for a uniform magnetic field the are spot travels at a uniform angular velocity, the frequency of the alternating current output from the secondary winding 20 is influenced by the strength of the magnetic field; that is to say, for any given tube the out put frequency will be determined by the distance between the successive anodes 9-9' and the strength of the magnetic field. Thus, for a tube of given geometry the output frequency may be varied within limits by varying the strength of the magnetic field. In the instance shown the strength of the field may be varied by means of a rheostat 28 in the supply line between a suitable source of direct current 29 and the coil ll.

The output frequency may be stabilized by the provision of a resonant circuit including the primary winding ii. In the instance shown a stabilizing circuit is provided by the condenser 22 connected across the end terminals of the primary winding ill. The parameters of the resonant circuit are so chosen that the natural frequency of this circuit is correlated to the angular velocity of the are spot, which, in turn, corresponds to the desired frequency of the alterhating current output. Thus, small variations in at a velocity corresponding to a frequency higher than the desiredfrequency, or of pulling the are spot into the region of one of the anodes if it has been retarded in its movement toward such region.

As previously stated, the output frequency can be varied by varying the magnetic field. Such variations in the magnetic field can be obtained by varying the current supplied to the coil H. When so altered it is desirable that the frequency of the resonant circuit, including the condenser 22, be correspondingly altered so that it will again be equal to the desired output frequency. For this purpose a small variable inductance 30 may be interposed in the resonant circuit or the capacitance of the condenser 22 may be varied, or both.

In the form of the invention shown, the envelope 6 is of glass. However, it will be apparent to those skilled in the art that the envelope may be of other suitable materials. For example, as in the construction of conventional mercury vapor rectifiers, the envelope may be of metal. provided the anodes 9 are suitably insulated therefom. However, the metal in this case should be non-magnetic, such as non-magnetic steel, or if of magnetic material the envelope should be so thin that it is readily saturated by the magnetic field imposed thereon so that upon saturation the material offers no further obstruction to the imposition of the magnetic field across the surface of the mercury pool.

While there has been herein described a preferred embodiment of the invention, other embodiments within the scope of the appended claims will be apparent to those skilled in the art from a consideration of the form shown and the teachings hereof. Accordingly, a broad interpretation of the claims, commensurate with the scope of the invention within the art, is desired.

What is claimed is:

1. A gaseous discharge device comprising an annular envelope containing a body of mercury providing an annular cathode, an annular anode spaced from said cathode, means for establishing an arc spot on said cathode, and means for imposing a magnetic field across the surface of said annular cathode effective to impart a retrograde motion to said are spot.

2. A gaseous discharge device comprising an annular envelope containing a body of mercury providing an annular cathode, an annular anode coaxial with said cathode and disposed in a plane spaced from and parallel with the surface of said cathode, means for establishing an are spot on said cathode, and means for imposing a magnetic field across the surface of said annular cathode eflective to impart a retrograde motion to said v are spot.

3. A gaseous discharge device comprising an annular envelope enclosing a discharge space and containing a body of mercury providing an annular cathode pool, a plurality of main anodes evenly distributed in said discharge space of said tube, means for establishing an arc spot on said pool, the distance between two successive main anodes being greater than the distance'of any of said anodes from said cathode, a magnet having a pair of annular pole pieces, one adjacent each side of said pool, the upper edges of the faces of said pole pieces lying in a plane closely adjacent the plane defined by the surface of said cathode pool, whereby said are spot is driven in a direction counter to the direction of rotation of a homopolar motor and the discharge is commutated successively from one of said anodes to another as said arc spot moves into proximity with first one anode and then another, and at least one auxiliary anode formulating said discharge when the arc spot is at points intermediate said main anodes.

4. A gaseous discharge tube for converting direct current into alternating current comprising an envelope providing an annular discharge space, an even number of main anodes positioned at regularly spaced intervals about said discharge space, a mercury-pool cathode coacting with said anodes, at least one auxiliary electrode, means for establishing an arc discharge between said auxiliary electrode and said cathode, means for shiftin said discharge through a predetermined orbital path whereby said discharge moves into proximity to said main anodes successively in the order in which said anodes are arranged about said discharge space, and whereby said discharge is commutated successively from one to another of said main anodes.

5. A gaseous discharge tube for converting direct current into alternating current comprising an envelope providing an annular discharge space, an even number of main anodes positioned at regularly spaced intervals about said discharge space, a mercuiy-pool cathode providing a continuous annular cathode surface coacting with said anodes, an annular auxiliary electrode, means for establishing anarc discharge between said auxiliary electrode and said cathode, means for shifting said discharge through a predetermined orbital path whereby said discharge moves into proximity to said main anodes successively in the order in which said anodes are arranged about said discharge space, and whereby said discharge is commutated successively from one to another of said main anodes.

6. A gaseous discharge tube for converting direct current into alternating current comprising an envelope providing an annular discharge space, an even number of main anodes positioned at regularly spaced intervals about said discharge space, a mercury-pool cathode providing a continuous annular cathode surface coacting with said anodes, at least one auxiliary electrode, means for establishing an arc discharge between said auxiliary electrode and said cathode, means for shifting said discharge at a predetermined velocity through a predetermined orbital path whereby said discharge moves into proximity to said main anodes successively in the order in which said anodes are arranged about said discharge space, and whereby said discharge is commutated successively from one to another of said main anodes.

7. A-gaseous discharge tube for converting direct current into alternating current comprising an envelope providing an annular discharge space, an even number of main anodes positioned at regularly spaced intervals about said discharge space, a mercury-pool cathode providing a continuous annular cathode surface coacting with said anodes, at least one auxiliary electrode,

means for establishing an arc discharge between said auxiliary electrode and said cathode, magnetic means for shifting said discharge through a predetermined orbital path whereby said discharge moves into proximity to said main anodes successively in the order in which said anodes are arranged about said discharge space, and whereby said discharge is commutated successively from one to another of said main anodes.

8. A gaseous discharge tube for converting direct current into alternating current comprising an envelope providing an annular discharge space, an even number of main anodes positioned at regularly spaced intervals about said discharge space, a mercury-pool cathode providing a continuous annular cathode surface coacting with said anodes, at least one auxiliary electrode, means for establishing an arc discharge between said auxiliary electrode and said cathode, means for establishing a magnetic field transverse to said annular discharge space for shifting said discharge at a predetermined velocity through a predetermined orbital path whereby said discharge moves into proximity to said main anodes successively in the order in which said anodes are arranged about said discharge space, and whereby said discharge is commutated successively from one to another of said main anodes.

9. A gaseous discharge device comprising an envelope containing a body of mercury providing a cathode, an anode spaced from said cathode, means for establishing an are spot on said cathode, and means for imposing a magnetic field across the surface of said cathode effective to impart a motion to said arc spot, the pressure within said envelope and the cathode-anode spacing being such that the motion of said are spot is retrograde motion.

10. A gaseous discharge device comprising an envelope containing a body of mercury providing a pool cathode having an elongated surface providing a closed path, an anode spaced from said cathode, means for establishing an are spot on said cathode, and means for imposing a magnetic field across the surface of said cathode effective to impart a motion to said arc spot around said closed path, the pressure within said envelope and the cathode-anode spacing being such that said are spot is driven in a direction counter to the direction of rotation of a homopolar motor.

11. A gaseous discharge device comprising an envelope containing a body of mercury providing a cathode, an anode spaced from said cathode a distance greater than a few millimeters, means for establishing an are spot on said cathode, and means for imposing a magnetic field across the surface 0! said cathode effective to impart a motion to said are spot. the pressure within said envelope being less than a centimeter of mercury, whereby the motion of said are spot is retrograde motion.

12. A gaseous discharge device comprising an envelope containing a body of mercury providing a cathode, an anode spaced from said cathode a distance less than a few millimeters, means for establishing an arc spot on said cathode, and means for imposing a magnetic field across the surface of said cathode effective to impart a motion to said arc spot, the pressure within said envelope being greater than a centimeter of mercury, whereby said are spot is driven in a direction counter to the direction of rotation of a homopolar motor.

CHARLES G. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 787,228 Steinmetz Apr. 11, 1905 877,026 Weintraub Jan. 21, 1908 1,137,964 Goddard May 4, 1915 1,262,491 Hewitt Apr. 9, 1918 1,264,420 Meyer Apr. 30, 1918 1,714,407 Smith May 21, 1929 FOREIGN PATENTS Number Country Date 308,978 Great Britain Aug. 5, 1930 Certificate of Correction Patent N 0. 2,473,826.

June 21, 1949.

CHARLES G. SMITH It is hereby certified that errors appear in the printed specification of the above numbered patent reqiuring correction as follows:

Column 5, line 24, for therefom read therefrom; column 6, line 3, claim 3, for

the word formulating read maintaining;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of' the case in the Patent Oflice.

Signed and sealed this 22nd day of November, A. D. 1949.

THOMAS F. MURPHY,

Assistant Oomm-iuioner of Patents. 

